1. Field of the Invention
This invention relates to optoelectronic devices, more particularly, to Vertical Cavity Surface Emitting Lasers (VCSELs).
2. Description of the Related Art
Use of optically transmitted signals in communication systems is dramatically increasing the throughput rate of data transfer. In typical network configurations, an electrical signal is converted into an optical signal by either a laser diode or a light emitting diode (LED). The optical signal is transported through a waveguide such as an optical fiber to an optical detector, which converts the optical signal into an electronic one.
A unit can be assembled that incorporates components for performing many of these functionalities into a single module. Such a module may comprise an integrated circuit, one or more light sources such as LED or laser diodes, and one or more optical detectors such as silicon, InP, InGaAs, Ge, or GaAs photodiodes. The optical detector is used to detect optical signals and transform them into electrical waveforms that can be processed by integrated circuitry in the IC. In response, optical signals are output by the light sources, which may be controlled by the circuitry in the IC. The optical detector(s) may be formed on a silicon, InP, InGaAs, Ge, or GaAs substrate while the optical source(s) are included on a GaAs, InGaAs, InP, InGaAsP, AlGaAs, or AlGaAsSb substrate. The integrated circuitry can be incorporated into either or both of the two semiconductor chips. The two chips may be bonded together, using for example, flip-chip or conductive adhesive technology.
In many cases, laser diodes are preferred over LEDs as light sources. The laser diode, for example, provides a higher intensity beam than the LED. Additionally, its optical output also has a narrower wavelength spectrum, which is consequently less affected by dispersion caused by transmission through the optical fiber. xe2x80x9cLaser diodexe2x80x9d is a general term that includes two broad types of semiconductor lasers. The first type of laser diode is an edge-emitting laser that emits light through an edge of an active region that comprises, for example, a p-n junction layer. The second type of semiconductor laser diode is a vertical cavity surface emitting laser (VCSEL).
A typical VCSEL comprises a plurality of layers of semiconductor material stacked on top of each other. A region centrally located within the stack corresponds to the active region comprising a p-n junction formed by adjacent p- and n-doped semiconductor layers. This active region is conventionally interposed between two distributed Bragg reflectors (DBRs), each DBR comprising a plurality of semiconductor layers with thicknesses selected so as to facilitate Bragg reflection as is well-known in the art.
The term xe2x80x9cverticalxe2x80x9d in Vertical Cavity Surface Emitting Laser pertains to the fact that the planar layers comprising the DBRs and the active region, when oriented horizontally, are such that a normal to the planes faces the vertical direction and light from the VCSEL is emitted in that vertical direction in contrast with horizontal emission emanating from a side of an edge-emitting laser. VCSELs offer several advantages over edge-emitting lasers, for example, VCSELs are typically much smaller than edge-emitting lasers. Furthermore, VCSELs produce a high intensity output. This latter advantage, however, can be negated if the emitted beam cannot be effectively captured and transmitted to an external location, e.g., via a waveguide. Typically, an optical coupling element such as a lens must be positioned adjacent to and aligned precisely with the VCSEL in order to achieve efficient optical coupling. This process reduces the cost effectiveness of using VCSELs in many instances, especially when a plurality of VCSELs are arranged in a one- or two-dimensional array.
Another advantage afforded by the VCSEL is increased beam control, which is provided by an aperture that is formed in one or more of the semiconductor layers. This aperture is conventionally formed by exposing the stack of semiconductor layers to water vapor to oxidize one of the layers. Initially outer edges of this semiconductor layer begin oxidizing; however, this oxidation progresses inward until the water vapor can no longer permeate the layer from the sides, wherein oxidation stop. Thus, a central region of the semiconductor layer remains un-oxidized. When the VCSEL is activated, current will flow through this central region and not through the surrounding oxide barrier. In this manner, the current flow is confined to a small portion of the active layer. Recombination of electrons and holes within this region causes light to be generated only within a small, localized area within the VCSEL. For the foregoing reasons, this aperture and the layer containing it are conventionally referred to in the art as a current confinement layer.
Disadvantageously, controlling the fabrication of the current confinement layer is particularly difficult. Vapor flow rates, temperature, and exposure time are among the many variables that affect the size and quality of the aperture that can be formed. Precise control of the dimensions of the aperture, upon which the size of the beam critically depends, is particularly problematic.
Accordingly, there is a need for improved optical coupling of the output light from the VCSEL to an external light-carrying medium such as waveguides. There is also a need for a more precise process for fabricating the current-confinement region within the VCSEL that largely defines its beam profile.
One aspect of the invention is a method of forming an optoelectronic module. The method comprises forming a plurality of optoelectronic devices on a substrate; removing at least a portion of the substrate, and mounting a fiber optic faceplate at the location of the removed substrate to receive light from the optoelectronic devices. In one embodiment, the method further comprises stabilizing the optoelectronic devices prior to mounting the fiber optic faceplate. Preferably, the step of stabilizing the optoelectronic devices comprises forming a temporary substrate over the optoelectronic devices. In one embodiment, forming the temporary substrate comprises filling interstices between optoelectronic devices with a filler material and attaching a temporary substrate to the filler material. The temporary substrate is preferably removed after the fiber optic faceplate is mounted. In another embodiment, forming the optoelectronic devices on the substrate comprises providing an etch stop layer between the optoelectronic devices and the substrate to protect the optoelectronic devices from etchants. In yet another embodiment, openings are etched through the substrate for light to pass from optoelectronic devices through the openings. The optoelectronic devices may comprise VCSELs and/or optical detectors.
In another aspect, a method of forming an optoelectronic module includes forming a plurality of optoelectronic devices on a substrate and removing at least a portion of the substrate. A substantially optically transmissive element is mounted at the location of the removed substrate to provide an optical path to the optoelectronic devices. The optoelectronic devices are stabilized prior to mounting the optically transmissive element by affixing a temporary substrate to the optoelectronic devices. The optoelectronic devices may comprise VCSELs and/or optical detectors.
In yet another aspect of the invention, a method of forming an optoelectronic module includes forming a Vertical Cavity Surface Emitting Laser (VCSEL) on at least one layer that is formed on a substrate and removing the substrate. The at least one layer is mounted to an integrated circuit (IC) chip and vias are formed in the at least one layer to provide electrically conducting pathways from the VCSEL to the IC chip. A faceplate is mounted above the VCSEL to receive light from the VCSEL.
In still another aspect, a method of forming an optoelectronic device comprises forming a multiplicity of VCSELs on a wafer and cutting the wafer into a plurality of pieces such that each piece contains a plurality of VCSELs. The pieces are mounted to respective IC chips and electrically connected to the VCSELs. In one embodiment, the method further comprises performing functionality tests on the pieces prior to mounting the pieces to respective IC chips. Preferably, the pieces are mounted to the IC chips by solder bonding, thermo-compression bonding, or conductive adhesive.